Invasive Plants of the United States

Canada thistle

Asterales > Asteraceae > Cirsiumarvense (L.) Scop.

Synonym(s): Californian thistle, creeping thistle, field thistle

Canada thistle is a tall, erect, spiny, perennial, herbaceous plant that grows to 4 ft. (1.2 m) tall. It has an extensive creeping rootstock. The leaves are lance-shaped, irregularly lobed, 2-6 in. (5-15 cm) long with prickly margins. The stems are ridged and hairy. The flowers are purple to white and can be up to 0.5 in. (1.8 cm) in diameter. Flowering occurs in late June to August. The small fruit, called achenes, are 1 to 1.5 in. (2.5-3.8 cm) long and have a feathery pappus which allows them to be dispersed further by wind. Numerous species of thistle occur in North America, and while some are invasive, many are native. Often the species are difficult to distinguish. Canada thistle can invade a variety of open habitats including prairies, savannas, fields, pastures, wet meadows and open forests. It forms dense stands which can shade out and displace native vegetation. Once established it spreads rapidly and is difficult to remove. Canada thistle is native to Europe and Asia and was accidentally introduced to North America in the 1600s.